Physical and Functional Interaction of the p14ARF Tumor Suppressor with Ribosomes

Abstract

Alterations in the p14ARF tumor suppressor are frequent in many human cancers and are associated with susceptibility to melanoma, pancreatic cancer, and nervous system tumors. In addition to its p53-regulatory functions, p14 ARF has been shown to influence ribosome biogenesis and to regulate the endoribonuclease B23, but there remains considerable controversy about its nucleolar role. We sought to clarify the activities of p14 ARF by studying its interaction with ribosomes. We show that p14 ARF and B23 interact within the nucleolar 60 S preribosomal particle and that this interaction does not require rRNA. In contrast to previous reports, we found that expression of p14 ARF does not significantly alter ribosome biogenesis but inhibits polysome formation and protein translation in vivo. These results suggest a ribosome-dependent p14 ARF pathway that regulates cell growth and thus complements p53-dependent p14 ARF functions.The INK4a/ARF locus on chromosome 9 is frequently altered in human cancer, and inherited INK4a/ARF mutations are associated with melanoma susceptibility in 20 -40% of multiple case melanoma families (1). This complex sequence encodes the melanoma tumor suppressor proteins, p16 INK4a and p14ARF from alternative spliced transcripts in different reading frames (2, 3). Both proteins are centrally involved in the regulation of cell cycle and apoptotic programs in response to oncogenic stimuli and are therefore frequently targeted in tumor development and progression (4). INK4a/ARF-deficient mice are tumor-prone (5-7), and those with melanocyte-specific expression of activated H-ras develop cutaneous melanomas with high penetrance (8). High density mapping on chromosome 9p in human melanomas has identified p14 ARF as the most commonly deleted INK4a/ARF gene (9), and individuals with altered ARF, but apparently wild type p16INK4a , are melanoma-prone (10 -13). p14 ARF interacts with the p53 negative regulator hdm2, and inhibits its p53-specific E3 3 ubiquitin ligase activity (14 -17). It has been proposed that ARF physically sequesters hdm2 in nucleoli, thus relieving nucleoplasmic p53 from hdm2-mediated degradation (18). Recent data, however, suggest that nucleolar relocalization of hdm2 is not required for p53 activation (19) and that the redistribution of ARF into the nucleoplasm enhances its interaction with hdm2 and its p53-dependent growth-suppressive activity (20, 21). Accordingly, increasing nucleolar localization of ARF reduces ARF p53-dependent functions and diminishes ARF-hdm2 complex formation (21). This current model of ARF function supports the concept that nucleolar disruption contributes to p53 signaling (22) because many stress signals perturb the nucleolus, causing the release of nucleolar proteins (including ARF, L11, L23, L5, and B23) that activate the p53 pathway (23-27).Nucleolar ARF, rather than residing in inactive "storage" (20, 21, 28), may regulate ribosome biogenesis by retarding the processing of early 47 S/45 S and 32 S rRNA precursors (29). These effects do not depend on...

Diffuse large B-cell lymphomas with CDKN2A deletion have a distinct gene expression signature and a poor prognosis under R-CHOP treatment: a GELA study

“…In contrast, the p14ARF protein inactivates the MDM2 protein and thereby stabilizes the p53 protein, but also suppresses ribosome biogenesis. 37 Therefore, genetic alterations of the CDKN2A locus may impair both p14ARF/p53 and p16INK4A/pRb pathways, providing a selective advantage in the development and progression of tumors. In the present study, molecular anatomy of the deletions determined clearly indicated that in almost all cases, both p14ARF and p16INK4A are inactivated.…”

ARF Suppresses Tumor Angiogenesis through Translational Control of VEGFA mRNA

“…S5), suggesting that the inhibition of VEGFA IRES activity may not be attributed to the defective rRNA maturation commonly observed in ARF-expressing and DKC1 m cells. p14 ARF was shown to inhibit global protein synthesis by interfering with polysome formation (40). Therefore, translational repression of VEGFA may merely reflect such global effects of ARF protein.…”

The pre-existing population of 5S rRNA effects p53 stabilization during ribosome biogenesis inhibition

“…While 28S, 5.8S, 5S rRNA and large ribosomal proteins constitute (LRPs) the 60S subunit, the 40S subunit is composed by 18S rRNA and small ribosomal proteins (SRPs). Several oncogenes, like c-Myc, are able to enhance the production of ribosomal components, by the stimulation of all the three RNA Polymerases [6–9], while tumor suppressors like p14 ARF or p53, are able to bind and inhibit the activity of several transcription factors like SL1 and UBF, needed for Polymerase I transcription, leading to ribosome biogenesis inhibition [10, 11]. On the other hand, several alterations in ribosome biogenesis, such as rRNA transcription inhibition or rRNA processing impairment, lead to a coordinated induction of p53 activity.…”